The present invention relates to wind measurement.
In particular, the present invention relates to a method and apparatus for wind measurement, wherein the position or velocity vector of a meterological sounding balloon is determined at appropriate time intervals utilizing a VLF radio navigation system.
Radio navigation systems have been known for use in wind measurement. One such type system, the very low frequency (VLF) radio navigation system, utilizes a ground station in conjunction with a sounding balloon. The sounding balloon is a free flying balloon carrying various meteorological instruments which are typically used for obtaining records of temperature, pressure and humidity in the upper atmosphere. The sounding balloon when used in the VLF navigation system also includes a radio receiver which can receive VLF signals transmitted from ground transmission stations. The VLF signals received by the sounding device are retransmitted by a telemetric link to the observation station, which then processes the signals whereby the phase of each of the VLF transmissions is measured.
Such navigation systems as have been described are typically referred to as hyperbolic navigation systems. In such systems, hyperbolic lines of position are produced by measuring the differences in time of transmission of the very low frequency radio signals from two or more synchronized transmitters located at fixed points. When such synchronized signals from the two stations are received, the difference in times of arrival at the receiver is constant on a hyperbola having the two transmitting stations as the foci. The measured time difference locates the receiver on one of the hyperbolic lines of position representing that time difference. Thus, if two transmitting stations each send out the synchronized signals, a receiver positioned so as to receive the synchronized signals will receive them at a time difference. The difference in time of reception of the two signals at the receiver is an indication of the difference in distance from the two stations. The locus of all points having the same difference is a hyperbola. The difference in phase of signals received from different transmitters can also be used to measure the difference in distance from a receiver positioned with respect to the two transmitters. In such cases, the hyperbolic lines of position represent constant phase difference lines with respect to the two transmitters. While there are many variations of the hyperbolic navigation system, one of the most common known such VLF navigations systems is the so called Omega system. In the Omega system, direct phase comparisons are made between the two transmitting stations. The system utilizes frequencies in the 10-15 kHz band. The transmissions from the stations are sequential and are followed by transmissions at frequencies slightly shifted from the first frequency.
In utilizing a VLF radio nagivation system for wind measurement, a sounding device is sent into the atmosphere. The sounding device is equipped with a radio receiver which can receive the VLF signal from the transmitting station. The signals received are then relayed to an observation station by means of a telemetric connection accomplished by use of a UHF link. Each of the transmitting stations transmits the VLF signal according to a time sharing system within the range of frequencies of 10-15 kHz. The pulse pattern of the VLF signal is identically repeated with constant time intervals between each transmission. Between the pulses there is usually a pause in order that the transmitted signal should have time to be attenuated after it is transmitted.
When utilizing two transmitting stations, the hyperbolic lines of position represent a constant phase difference. When information is received from a given receiver located with respect to the two stations, the determined phase difference will locate the receiver on a given hyperbolic line. However, two similar parts of the hyperbola exist whereby an ambiguity is present as to the exact poistion of the receiver. This ambiguity is resolved by the use of an additional station whereby the three stations form two pairs of stations. Hyperbolas are then formed between each pair of stations and a fix can be obtained at the intersection at the particular hyperbolic lines.
Position finding in the VLF system is therefore achieved by measuring the phase differences between the signals transmitted by the three stations. Between any two of these stations, there exists the hyperbolic line of positions with the two stations at its focal points. However, since generally the stations are separated by great distances, the same phase difference hyperbolic lines will be repeated at numerous points along the line connecting the two stations. Each of these points are spaced by one half the wave length. The area between two hyperbolas corresponding to a zero degree phase difference is generally referred to as a lane. By measuring the phase differences of two stations, the position of the receiver with reference to the respective lanes can be determined. However, in actual navigational use, such determination will be ambiguous since two possibilities occur on the lane. In wind measurement, however, this ambiguity of lanes does not cause any difficulty because the starting point is known. Therefore, the correct lane can be found by counting the lands which the meteoroligical sounding device has traversed since its start. As a general rule, a meteorological sounding device moves within a small enough area to allow the width of the lanes and the angle between them to be assumed to be constant.
In utilizing a VLF system for wind measurement, the accuracy of the VLF system can be improved by utilizing a differential measurement. Errors are frequently introduced as a result of faults in the propagation of the VLF signal. Such errors can be corrected by utilizing a fixed point of observation on the ground as a VLF signal receiver, in addition to the sounding device. Thus, when a signal is transmitted, the signal is simultaneously received both at the fixed point of observation on the ground as well as at the meteorological sounding device. Any such faults in propagation cause approximately equal changes of phase at both the sounding device and the ground observation point if the distance between the sites is less than about 400 km. Correction of such propagation errors can then be achieved by applying the measurements made at the fixed point of observation on the ground to the measurements made at the meteorological balloon. In this manner, phase errors introduced by propagation can be removed from the measurement made at the meteorological balloon and the phase difference at the meteorological balloon will therefore only be as a result of the position of the meteorological sounding device.
When a meteorological sounding device is observed, it is usually desired to find its position or velocity vector at one minute intervals. It is therefore possible to receive several pulses from each of the stations being utilized.
Several different VLF position finding receivers are known in the art. By way of example, such systems have been described in the following literature: J. P. van Etten, "Navigation Systems: Fundamentals of Low-And Very-Low-Frequency Systems", Electrical Communication, Vol. 45, No. 3, 1970, p. 192-212; "LO-GATE III W-3 Technical Description and System Description", Beukers Laboratories Inc.; and R. H. Woodward, J. A. Pierce, W. Palmer, and A. A. Watt, "Omega-- A World-Wide Navigational System", Pickard and Burns Electronics, 1966, p. 364.
In the VLF navigation devices of the prior art, phase determination is achieved by means of a phase locked loop. This method of determining the phase has numerous drawbacks. Firstly, in order to maintain phase locking, it is necessary that the receivers obtain an extremely good signal since phase locking is difficult with poor signal to noise ratios. This problem is especially of concern in wind measurement where many signals are transmitted in the same telemetric channel which therefore introduces numerous harmonics and interfering frequencies from intermodulation. This reduces the signal to noise ratio and makes good phase locking even more difficult. Additionally, narrow band filters have to be utilized in the processing equipment and these introduce additional phase errors. The receivers utilized in the prior art systems have been extremely complex and costly because of the many requirements placed upon such receivers. Such requirements were necessary since the band widths of the phase lock loops affect the phase following capacity as well as the speed of locking. Furthermore, with prior art systems, it was not possible to measure all of the information contained in the signals, especially because of the poor signal to noise ratios which frequently occur.